1. Field of the Invention
The present invention relates to a signal processing apparatus for processing a signal which is obtained from a photosensor whose impedance varies with changes in quantity of light received thereby.
It also relates to a binary encoder circuit for converting an input signal including a A.C. component output from the signal processing apparatus as mentioned above into a binary signal corresponding to variations in the A.C. component.
2. Description of the Prior Art
Nowadays photosensors are widely used for converting changes in quantity of received light into electrical signals corresponding to changes in brightness in image reading apparatus and optical coordinate inputting apparatus such as bar code readers, optical rotary encoders, facsimile devices, and image scanners.
Such a photosensor connected in series with a resistor is interposed between a power source and ground, and variations in the impedance of the photosensor in accordance with changes in quantity of received light is converted into an electrical signal and output as variations in the voltage divided by the photosensor and resistor.
The impedance of a photosensor does not become infinity even when no light is incident thereon and a dark current then flows therethrough. The quantity of light impinging on a photosensor consists of the reflected light from a printed object of inspection such as a bar code and the ambient disturbance light. Therefore, the current flowing through the photosensor includes an A.C. component as the signal for the changes in the reflected light and a D.C. component superposed thereon due to the dark current or ambient disturbance light. The D.C. component widely varies with the ambient brightness, quantity of light emitted from a light source impinging on the surface of a bar code or the like, reflectivity of the bar code surface, magnitude of the dark current characteristic of the photosensor, and so on.
Therefore, to extract the A.C. component as the signal while blocking up such a D.C. component, a signal processing system of an A.C. coupling type, for example, has so far been practiced. FIG. 19 is a circuit diagram of a conventional signal processing apparatus of such an A.C. coupling type. In FIG. 19, the light emitted from an LED 1 to impinge on a bar code surface is reflected by the bar code surface and received by a phototransistor 2 as a photosensor. The phototransistor 2 and a resistor 3 connected in series are interposed between a power supply terminal and ground and a junction of the phototransistor 2 and the resistor 3 is connected through a direct current blocking capacitor 4 with an amplifier 5 or the like in the stage succeeding thereto.
With such arrangement, the D.C. component widely varying with ambient brightness and other is blocked up by the capacitor 4 and only the A.C. component as the signal is supplied to the amplifier 5 or the like.
Generally a bar code reading apparatus is adapted such that the bar code reading is performed by having a bar code surface on which predetermined character information has been recorded scanned with light and changes in quantity of the reflected light obtained by such a scanning on the bar code surface are converted into an electrical signal by means of a photosensor.
The electrical signal output from the photosensor as the result of the scanning on the bar code surface is composed of a low-intensity signal portion, or an A.C. component with small amplitudes changing in one direction, and a D.C. component superposed thereon with considerable intensity due to ambient disturbance light and dark current in the photosensor. Therefore, in order to read the bar code, there is provided a binary encoder circuit for converting the small changes in the A.C. component into a binary signal.
An example of a conventional binary encoder circuit is shown in FIG. 21. In the circuit of FIG. 21, a signal input terminal 1 supplied with an input signal is connected with a power supply V through a resistor 2 and with the negative input of a comparator 3, and further connected with the anode of a first diode 4 and the cathode of a second diode 5. The cathode of the first diode 4 and the anode of the second diode 5 connected with each other are grounded through a capacitor 6 and also connected with the positive terminal of the comparator 3 through a resistor 7. The output terminal of the comparator 3 is connected with a signal output terminal through a resistor 9. In the above, the resistors 7 and 9 are for applying a positive feedback to the comparator 3 for providing a hysteresis voltage therefor.
With such arrangement, if an input signal as indicated by the solid line in FIG. 22(a) is supplied to the signal input terminal 1, the forward voltage drop V.sub.F in the first diode 4 is reduced from the variation when the A.C. component is on the rise, and the forward voltage drop V.sub.F in the second diode 5 is reduced from the variation when the A.C. component is on the fall, and further, the signal is smoothed to a certain degree by the capacitor 6, and consequently, the signal as indicated by the one-dot chain line in FIG. 22 (a) is supplied to the positive input of the comparator 3. Thus, a binary encoded signal as shown in FIG. 22 (b) in which "H" and "L" output levels are switched to each other each time the relative magnitude of the signals supplied to the positive and negative inputs is reversed is output from the output terminal of the comparator 3 to the signal output terminal 8.
Now, in the above described signal processing apparatus of an A.C. coupling type, until a D.C. potential difference across the capacitor 4 developed by the D.C. component is discharged, the mean value of the A.C. component appearing at the stage succeeding to the capacitor 4 varies as indicated by the dotted line in FIG. 20(b). Therefore, when such an A.C. component has been binary encoded with a value taken as reference, there has been a problem such as that a binary signal having different widths from the A.C. component given to the capacitor 4 as shown in FIG. 20(a) has been produced.
Besides, since the signal processing apparatus of the A.C. coupling type have required a large dynamic range corresponding to a large change in the D.C. component, and since such a large dynamic range has not been obtained in apparatus using a power source at a low voltage such as a battery, there has been dissatisfaction with such apparatus.
Accordingly, the present invention was directed to solve the above mentioned problem in the conventional signal processing apparatus, and it has as its first technical problem the provision of a signal processing apparatus for outputting a signal corresponding to changes in the impedance of a photosensor superposed on a predetermined reference voltage.
Also, the present invention was directed to solve the above mentioned problem in the conventional signal processing apparatus, and it has as its second technical problem the provision of a signal processing apparatus whereby only the A.C. component corresponding to changes in the impedance of a photosensor is amplified and output as a signal.
Further, in the above described conventional binary encoder circuit, since the threshold levels are established by the forward voltage drop V.sub.F in the first and second diodes 4 and 5, in the case where the magnitude of variation in the A.C. component is smaller than the forward voltage drop V.sub.F, there occurs no reversal of relative magnitude of the signals supplied to the positive and negative inputs of the comparative 3, so that the comparator 3 is unable to detect a signal. Even if it be able to, the pulse widths of the obtained binary signal hardly agree with the pulse widths of the input signal and a large error is produced. Furthermore, since the forward voltage drop V.sub.F in the first and second diodes 4 and 5 is relatively high as approximately 0.6 V, the magnitude of variation of the A.C. component must be at least 1.2 V in order that the input signal is correctly binary encoded. In a bar code reader, the amplitude of the electrical signal from the photosensor varies as much as 3 to 10 times according to conditions such as reflectivity of the paper surface or the angle of incidence of the reflected light. Therefore, in order that the input signal is binary encoded correctly, 3.6 to 12 V or so of the dynamic range of the input signal is required. Hence, a voltage as high as 5 to 15 V or so is required of the power supply voltage of the bar code reader. Specifically, in the case of a portable type obtaining the power voltage from a dry battery incorporated therein, the number of the dry cells to be connected in series becomes large to obtain such a high power voltage, and this becomes a big problem in practical use of such a type.
Besides, since the foward voltage drop V.sub.F in the first and second diodes 4 and 5 varies with temperature, there is such a problem that the magnitude of variation in the A.C. component of the input signal which can be converted to a binary signal is inconstant, or the pulse widths of the converted binary signal are variable.
Accordingly, the present invention was made in view of the above described conditions of the conventional binary encoder circuit, and it has as its third technical problem the provision of a binary encoder circuit capable of correct binary encoding of an input signal whose magnitude of variation is small.